1. Field of the Invention
The invention relates to a hydraulic control apparatus for an automatic transmission. More particularly, the invention relates to a hydraulic control apparatus and a hydraulic control method that individually control engagement pressures of a plurality of hydraulic engagement devices that are used to achieve a plurality of shift speeds using respective solenoid valves, and control engagement of a lock-up clutch using the solenoid valves.
2. Description of the Related Art
In some automatic transmissions, a plurality of shift speeds at which the speed ratio are different from each other are achieved by selectively engaging a plurality of hydraulic engagement devices. Japanese Patent Application Publication No. JP-A-2000-249219 describes an example of a hydraulic control apparatus that individually controls engagement pressures of a plurality of hydraulic engagement devices using respective solenoid valves in such automatic transmissions. Japanese Patent Application Publication No. JP-A-2001-248725 describes an automatic transmission that includes a hydraulic power transmission device with a lock-up clutch. In the automatic transmission, the engagement pressures of the hydraulic engagement devices and the torque capacity of the lock-up clutch are controlled using one solenoid valve. That is, a control hydraulic pressure output from the solenoid valve is selectively supplied to the engagement devices or the lock-up clutch using a relay valve (switching valve), whereby the engagement pressures of the engagement devices or the torque capacity of the lock-up clutch can be selectively controlled. After the engagement device is completely engaged, supply of the control hydraulic pressure to the engagement device stops, and a line hydraulic pressure is supplied to the engagement device. Therefore, the torque capacity of the lock-up clutch can be controlled using the control hydraulic pressure output from the solenoid valve.
For example, the technology described in Japanese Patent Application Publication No. JP-A-2001-248725 may be applied to the automatic transmission described in Japanese Patent Application Publication No. JP-A-2000-249219. More specifically, in the case where a solenoid valve controls the engagement pressure of the hydraulic engagement device that is engaged at the first forward speed, the solenoid valve may be also used to control the torque capacity of the lock-up clutch. However, when the lock-up clutch is engaged even at the first forward speed, supply of the control hydraulic pressure to the engagement device needs to be stopped, and the line hydraulic pressure needs to be supplied to the engagement device. In this case, when the shift speed is changed from the first forward speed to another forward speed, the engagement device needs to be disengaged by supplying the control hydraulic pressure to the engagement device again, and stopping supply of the line hydraulic pressure to the engagement device. This requires a complex control scheme, and may adversely affect shift response. Actually, the torque capacity of the lock-up clutch cannot be controlled at the first forward speed. In the case where another solenoid valve controls the engagement pressure of the engagement device that is engaged at a forward speed other than the first forward speed and the solenoid valve is used to control the torque capacity of the lock-up clutch, the torque capacity of the lock-up clutch cannot be controlled at the forward speed.
In this specification, the phrase “the hydraulic pressure is supplied” signifies “the hydraulic pressure is applied” or “hydraulic oil that is controlled by the hydraulic pressure is supplied”.